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CHAP. 99. (97)—CONCERNING THE CAUSE OF THE FLOWING
AND EBBING OF THE SEA.

Much has been said about the nature of waters; but the
most wonderful circumstance is the alternate flowing and
ebbing of the tides, which exists, indeed, under various forms,
but is caused by the sun and the moon. The tide flows
twice and ebbs twice between each two risings of the moon,
always in the space of twenty-four hours. First, the moon
rising with the stars1 swells out the tide, and after some time,
having gained the summit of the heavens, she declines from
the meridian and sets, and the tide subsides. Again, after she
has set, and moves in the heavens under the earth, as she
approaches the meridian on the opposite side, the tide flows
in; after which it recedes until she again rises to us. But
the tide of the next day is never at the same time with that of
the preceding; as if the planet was in attendance2, greedily
drinking up the sea, and continually rising in a different place
from what she did the day before. The intervals are, however,
equal, being always of six hours; not indeed in respect of any
particular day or night or place3, but equinoctial hours, and
therefore they are unequal as estimated by the length of common
hours, since a greater number of them4 fall on some certain days or
nights, and they are never equal everywhere
except at the equinox. This is a great, most clear, and even
divine proof of the dullness of those, who deny that the stars
go below the earth and rise up again, and that nature presents the same face in the same states of their rising and
setting5; for the course of the stars is equally obvious in the
one case as in the other, producing the same effect as when
it is manifest to the sight.

There is a difference in the tides, depending on the moon,
of a complicated nature, and, first, as to the period of seven
days. For the tides are of moderate height from the new
moon to the first quarter; from this time they increase, and
are the highest at the full: they then decrease. On the
seventh day they are equal to what they were at the first
quarter, and they again increase from the time that she is at
first quarter on the other side. At her conjunction with
the sun they are equally high as at the full. When the
moon is in the northern hemisphere, and recedes further
from the earth, the tides are lower than when, going towards
the south, she exercises her influence at a less distance6.
After an interval of eight years, and the hundredth revolution of the moon, the periods and the heights of the tides
return into the same order as at first, this planet always
acting upon them; and all these effects are likewise increased by the
annual changes of the sun7, the tides rising
up higher at the equinoxes, and more so at the autumnal
than at the vernal; while they are lower8 about the winter
solstice, and still more so at the summer solstice; not
indeed precisely at the points of time which I have mentioned, but a
few days after9; for example, not exactly at
the full nor at the new moon, but after them; and not
immediately when the moon becomes visible or invisible, or
has advanced to the middle of her course, but generally
about two hours later than the equinoctial hours10; the effect
of what is going on in the heavens being felt after a short
interval; as we observe with respect to lightning, thunder,
and thunderbolts.

But the tides of the ocean cover greater spaces and produce
greater inundations than the tides of the other seas; whether
it be that the whole of the universe taken together is more
full of life than its individual parts, or that the large open
space feels more sensibly the power of the planet, as it moves
freely about, than when restrained within narrow bounds.
On which account neither lakes nor rivers are moved in the
same manner. Pytheas11 of Massilia informs us, that in
Britain the tide rises 80 cubits12. Inland seas are enclosed
as in a harbour, but, in some parts of them, there is a more
free space which obeys the influence13. Among many other
examples, the force of the tide will carry us in three days
from Italy to Utica, when the sea is tranquil and there is no
impulse from the sails14. But these motions are more felt
about the shores than in the deep parts of the seas, as in the
body the extremities of the veins feel the pulse, which is the
vital spirit, more than the other parts15. And in most estuaries, on account of the unequal rising of the stars in each
tract, the tides differ from each other, but this respects the
period, not the nature of them; as is the case in the Syrtes.

1 "Mundo;" the heavens or visible firmament, to which the stars and
planets appear to be connected, so as to be moved along with it.

6 It is scarcely necessary to remark, that both the alleged fact and the
supposed cause are incorrect. And this is the case with what our author
says in the next sentence, respecting the period of eight years, and the
hundred revolutions of the moon.

7 "Solis annuis causis." The circumstances connected with the
revolution of the sun, acting as causes of the period and height of
the tides,
in addition to the effect of the moon.

14 The effect here described could not have depended upon the tides,
but upon some current, either affecting the whole of the Mediterranean,
or certain parts of it. See the remarks of Hardouin in Lemaire.

15 Pliny naturally adopted the erroneous opinions respecting the state
of the blood-vessels, and the cause of the pulse, which were universally
maintained by the ancients.

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